The first land-walking vertebrates may have emerged from salty estuaries | Science News

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The first land-walking vertebrates may have emerged from salty estuaries

An analysis casts doubt on views that the ancient creatures arose in freshwater

By
5:29pm, May 30, 2018
tetrapod illustration

SALTY SCENE  The planet’s earliest four-footed vertebrates called tetrapods (illustrated) lived in the brackish waters of an estuary or delta, new research suggests.

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Earth’s earliest land-walking vertebrates didn’t paddle about in freshwater lakes or rivers. Instead, these four-footed creatures, which appeared about 375 million years ago, lived in the brackish waters of an estuary or delta, researchers report online May 30 in Nature.

Early tetrapods, such as Ichthyostega and Acanthostega, lived an amphibious existence between land and sea: They had feet, but also gills and tails made for swimming. A new study by paleontologist Jean Goedert of Université Lyon in France and colleagues suggests that the animals also could have tolerated rapid changes in salinity, such as is found in an estuary.

The researchers analyzed sulfur and oxygen isotopes — forms of these elements with the same number of protons, but different masses — in 51 fossilized tetrapod bones from locations in what’s now Greenland and China. Compared with freshwater, seawater has a higher ratio of the sulfur-34 isotope relative to sulfur-32. The tetrapod bones tended to show elevated sulfur-34, the researchers report, suggesting that the creatures spent at least some time in seawater. But oxygen isotope analyses of the bones show that freshwater was also present, arguing against a purely salty environment such as an ocean.

The results challenge a long-held view that the earliest tetrapods emerged from freshwaters, such as rivers or lakes. In 1929, the first Ichthyostega fossils were found in a series of red sandstone layers in eastern Greenland that geologists once thought had been deposited in a freshwater environment. But later discoveries of tetrapod fossils found associated with known marine species suggested that the early walkers may have lived in saltier waters than once thought.

An ability to tolerate different salinity environments could have helped tetrapods — a group that includes today’s amphibians, reptiles and mammals — survive a mass extinction of ocean-dwellers that occurred by the end of the Devonian Period about 359 million years ago, the researchers say.

Citations

J. Goedert et al. Euryhaline ecology of early tetrapods revealed by stable isotopes. Nature. Published online May 30, 2018. doi:10.1038/s41586-018-0159-2. 

Further Reading

D. Garisto. The wiring for walking developed long before fish left the sea. Science News. Vol. 193, March 17, 2018, p. 10.

S. Milius. Preteen tetrapods identified by bone scans. Science News. Vol. 190, October 15, 2016, p. 12.

S. Milius. Hightailing it out of the water, mudskipper style. Science News. Vol. 190, August 6, 2016, p. 13.

S. Perkins. Amphibious ancestors. Science News. Vol. 169, June 17, 2006, p. 379.

R. Monastersky. Out of the swamps. Science News. Vol. 155, May 22, 1999.

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